Numerous types of proportional pneumatic flow control valves are presently available which accurately control flow over a wide range of flow control applications. Two broad categories of such devices can be identified that included balanced valve designs, and unbalanced valve designs.
Balanced values are, in general, maintain the controlled fluid flow rate at a relatively constant rate (i.e., “balanced”) in spite of pressure changes. Used in fluid distribution systems (most commonly for water and air distribution), such balanced values are calibrated to a specific range, and their accuracy is dependent upon the total range of capable flow, which is otherwise known as the “Full Scale” of the valve. A well designed system with balanced values attempts to achieve a constant flow throughout the whole system that corresponds to a narrow range of flow rates around what was specified for the design of the system. While such designs can be very efficient in terms of minimizing pressure drop across the vales, if the correct balancing of the system is not established an unequal distribution of the flow results such that there will be a surplus flow in some of the terminals and an inadequate flow in others. The result of this will be that desired distribution cannot be assured throughout the system. In practice, unless the system is correctly balanced system by manipulation of the piping and/or correctly balancing all values in the distribution system, the wide swings in flow can be generated with increased pressure. To control such a system with PID controllers to eliminate such wide swings that are inherent within the system remains very difficult.
Unbalanced valves are (usually) simpler designed, smaller valves with low pressure drops across the valve. Advantages include a simpler design with fewer potential leak paths at the seat and a lower capital cost. Disadvantages include a limited sizes, since with a large unbalanced valve the forces needed to seat and hold the flow often becomes impractical. A general solution is to maintain holes through the plug in order to achieve an easier shut off as the plug does not have to overcome static forces. However, such a solution creates an additional leak path and at a cost that is generally higher.
In trim control applications, the selection of balanced or unbalanced designs is not entirely straightforward, and will ultimate be based upon the selection and design of the actuator. The actuator design is based on the thrust force which is required to open and close the valve, with the returning forced being supplied by either a spring or diaphragm. Selection of an actuator and spring range must be capable of handling that thrust force which is available in the selected size of valve.
Consequently, there is a need for a balanced valve mechanism that is capable of provide the same flow rate at wide range of pressure drops that is mechanically simple and low-cost.